1. Field of the Invention
The present invention relates to a liquid crystal display device for regulating the alignment direction of a liquid crystal molecule at the time of driving by using a polymer, a manufacturing method of the same, and a liquid crystal display device's substrate used for the same.
Besides, the invention relates to a liquid crystal display device and a manufacturing method of the same, and particularly to a liquid crystal display device in which an uneven display can be reduced and a manufacturing method of the same.
2. Description of the Related Art
Conventionally, as an active matrix type liquid crystal display device, a twisted nematic (TN) mode is widely used in which a liquid crystal material having a positive dielectric anisotropy is aligned to be horizontal with respect to a substrate surface in a dark state and to be twisted 90 degrees between opposite substrates.
This TN mode liquid crystal display device has a problem that its visual angle characteristics are poor, and various studies have been carried out to improve the visual angle characteristics. Then, as a mode replacing the TN mode, a multi-domain vertical alignment (MVA) mode has been developed. In the MVA mode, a liquid crystal material having a negative dielectric anisotropy is vertically aligned, and by alignment regulating structures, such projections or slits, provided on a substrate surface, inclination directions of liquid crystal molecules at the time of voltage application are regulated in plural directions without performing a rubbing treatment to an alignment film. The MVA mode liquid crystal display device is greatly improved in the visual angle characteristics as compared with the TN mode.
Although the MVA mode liquid crystal display device has superior visual angle characteristics as described above, since the projections or the slits for regulating the alignment are provided, the aperture ratio is inevitably lowered. Thus, the conventional MVA mode liquid crystal display device has a problem that the transmission factor is low as compared with the TN mode liquid crystal display device and its display causes a feeling of dark. Its main cause is that portions above the alignment regulating structures become the boundaries of alignment division to generate dark lines, and the transmission factor becomes low. In order to improve the transmission factor, the arrangement intervals of the alignment regulating structures have only to be made sufficiently wide. However, in that case, since the alignment regulating structures becomes few in number, even if a predetermined voltage is applied to a liquid crystal, it takes a long time for the alignment to become stable, and the response speed becomes low.
Further, it can not be neglected that the formation itself of the minute and fine projections or slits complicates the manufacturing process and increases the manufacturing cost.
Then, in order to realize the MVA mode liquid crystal display device which has high luminance and enables high speed response, a method has been proposed in which the alignment direction of a liquid crystal molecule at the time of driving is regulated by using a polymer. In this method, a liquid crystal material in which a liquid crystal and a polymerizable component such as a monomer or an oligomer are mixed is sealed between two substrates. As the polymerizable component, a material which is polymerized by light or heat is used. In a state where a predetermined voltage is applied between the substrates to incline the polymerizable component, UV light irradiation or heating is performed to polymerize the polymerizable component and the polymer is formed. By the polymer formed in the vicinity of the surface of the substrate, even if the voltage application is removed, the liquid crystal layer in which a predetermined alignment direction and a pre-tilt angle are regulated can be obtained. Thus, a rubbing treatment of an alignment film becomes unnecessary. As stated above, when the method for giving the predetermined alignment direction and pre-tilt angle to the liquid crystal molecule by the polymer is used, it becomes possible to provide the MVA mode liquid crystal display device which has high luminance and enables high speed response. Incidentally, for further details, please refer to the specification of Japanese Patent Application (Japanese Patent Application No. 2001-98455 and No. 2001-264117) by the applicants of the present application.
FIG. 42 shows a display region of a conventional MVA mode liquid crystal display device. A liquid crystal material in which a monomer is mixed is injected through a liquid crystal injection port 12 formed at one end part of a panel. While the injected liquid crystal material diffuses in a narrow cell gap, the distribution of the monomer becomes irregular in a display region 10. Especially, in regions β in the vicinities of two corners at the side opposite to the liquid crystal injection port 12, the concentration of the monomer becomes low as compared with another region α. Thus, in the regions β, a pre-tilt angle of a liquid crystal molecule obtained after a polymer is formed by irradiation of UV light becomes larger than that in the other region α. Here, the pre-tilt angle is an inclination angle of a liquid crystal molecule with respect to a substrate surface in a state where a voltage is not applied to a liquid crystal layer. That is, when the pre-tilt angle is 90°, the liquid crystal molecule is aligned vertically to the substrate surface.
FIG. 43 shows a luminance distribution on line A-A′ of a display screen of the liquid crystal display device shown in FIG. 42. The horizontal axis indicates position on the line A-A′, and the vertical axis indicates luminance. A left end part of the display region 10 on the line A-A′ is denoted by A0, a boundary between the region a and the region β is denoted by A1, and a right end part of the display region 10 is denoted by A2. Incidentally, this liquid crystal display device has a normally black mode, and it is assumed that the same gradation is displayed on the whole display region 10. As shown in FIG. 43, the almost uniform luminance distribution is obtained in the region α, however, in the region β, as compared with the region α, the luminance is lowered since the pre-tilt angle of the liquid crystal molecule is larger than that in the region α. Thus, an uneven luminance is seen on the display screen.
Besides, in the conventional color liquid crystal display device, when a half tone (gray scale) is displayed, coloring is seen. That is, at the change of the gradation from white to black, the chromaticity is changed. This phenomenon indicates that a different color is reproduced in not only in achromatic color but also in chromatic color, and there occurs a problem that a desired display image can not be obtained. Its cause is that since wavelengths of light transmitting through respective colors of color filter (CF) resin layers are different from one another, the substantial magnitudes of retardations including the liquid crystal layer are different among the respective colors, and the transmission characteristics (T-V characteristic) are different among the respective colors.
As measures to the above problem, a method called multi-gap is proposed in which a cell gap is changed for each pixel different in color. However, the manufacture in which the cell gap is controlled for each pixel has a problem that the process becomes complicated and the manufacturing cost is increased.
As other measures, there is a method in which an input signal is converted by a signal conversion element such as a scaler IC, and the T-V characteristics for each color are adjusted. However, the scaler IC including a frame memory is expensive and lacks versatility.
Besides, in an MVA mode liquid crystal display device using a method for giving a pre-tilt angle by using a polymer structure, as shown in FIG. 44, there is a case where an uneven display 100 occurs in the vicinity of a corner part 50 opposite to a liquid crystal injection port 12 in a half tone display. FIG. 44 is a schematic view showing the conventional liquid crystal display device. Thus, a technique of reducing the uneven display 100 has been awaited.